To measure the in vitro BLI signal intensity, both control and transduced cells were equally seeded (60 000 cells per well in a 24-well plate) and measured the next day after 5 min incubation at 37 °C with D-luciferin (Promega BeNeLux, Leiden, The Netherlands; 150 μg/l dissolved in PBS).
For in vivo BLI measurements, D-luciferin (126 mg/kg dissolved in PBS (15 mg/ml)) was injected intraperitoneally and acquisition of consecutive frames was started until maximum signal intensity was reached. Image acquisition numbers and times varied between 10 and 15 frames of 30-60 s each, depending on the optimal acquisition settings in function of the signal intensity intrinsic to the grade of lung metastasis. We measured the photon flux per second (p/s) through a region of interest (2.9 cm × 1.8 cm) covering the lungs.